Field of the Invention
The present invention relates to drilling and thread forming fasteners and more particularly to a drilling and thread forming fastener capable of manufacture by a forging process.
In the art of fastener manufacturing, a type of threaded fastener having the capabilities of drilling a pilot hole and forming threads for entry of the fastener, all combined in the same operation is well known. It is also known to manufacture these fasteners by employing a pair of dies which are effective to forge the drilling point into a configuration which is best suited for the drilling operation in a particular material, or for a particular size fastener. One such a fastener is shown in U.S. Pat. No. 3,395,603 issued Aug. 6, 1968, to E. J. Skierski and assigned to the assignee of the present invention.
While many of these drilling and thread forming fasteners have gained commercial acceptance, including the above cited Skierski fastener configuration, areas have been uncovered which are considered to be subject to improvement, to provide both increased utility of the particular fastener, and ease of manufacturing with a decrease in cost per fastener.
For instance it has been found that many of the prior art drilling and thread forming fasteners are not adaptable to performing their function in heavier gauge materials, that is material of a thickness greater than five-sixteenths (5/16) of an inch. While some of the fasteners disclosed in the prior art have the capability to perform this drilling function, the high heat generated by the drilling operation in addition to the increase drilling time, in many applications, render the use of the self-drilling and thread forming fasteners undesirable.
In the area of manufacture, many of the prior art drilling and thread forming fasteners are constructed having an outer pilot end surface which in cross section contains a number of sharp corners or sharp recesses which are designed into the drilling tip. Such sharp corners are detrimental when the fastener is manufactured by a forging process employing a female die to produce the fastener pilot end in that they create stress areas in the female die. Over a period of time, the stress areas are subject to cracking and will eventually result in die breakage of a premature nature. Thus, dies employed to manufacture that type of fastener having a plurality of sharp external surfaces which are subject to replacement after a somewhat limited number of cycles of use.
It is therefore the object of the present invention to provide a drilling and thread forming fastener which is particularly adopted for use in heavier gauge materials for instance those materials having a thickness greater than a five-sixteenths (5/16) of an inch.
A further object of the invention is to provide a drilling and thread forming fastener of the type discussed, wherein sharp corners in the fastener pilot end which are produced by sharp recesses in a forging die are reduced, or eliminated, to allow manufacture of the fastener by the forging process employing a die having few, if any, sharp recesses on the forming surface.